The old struct config_param remains only for top-level string options.

API simplification.  We can now avoid abusing a "size_t*" as
additional return value.

Prepare for adding state.

Prepare to migrate away from GLib.  Currently, we're still using GLib
as a backend.

Replaces GLib's GError.

Push the new mode to the filter instead of accessing global variables
through replay_gain_get_real_mode().

Eliminate support for reverse endian samples from the MPD core.  This
moves a lot of complexity to the plugins that really need it (only
ALSA and CDIO currently).

The other audio_format attributes are not used.

The ReplayGain filter clamped the gain to max. 100 % even if the
algorithm determined the signal needed a boost. That would result in any
such tracks being played with too low volume, effectively defeating the
purpose of the filter.

Add an option for each audio output which enables the use of the
hardware mixer, instead of the software volume code.

This is hardware specific, and assumes linear volume control.  This is
not the case for hardware mixers which were tested, making this patch
somewhat useless, but we will use it to experiment with the settings,
to find a good solution.

Apply the replay gain in the output thread.  This means a new setting
will be active instantly, without going through the whole music pipe.
And we might have different replay gain settings for each audio output
device.